1. Field of the Invention
The present invention relates to acrylic polymers containing pendant tin moieties and their use in the long-term inhibition of algae growth on building materials such as conventional asphalt shingles.
2. Description of Related Art
Pigmented granules, both natural and artificially color-coated, find extremely wide use in roofing and siding materials, such as in granule-surfaced bituminous roll roofing and asphalt shingles. The granules, which are partially imbedded in one surface of asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and decorative exterior surface. It is also preferred that the roofing granules provide resistance to ultraviolet radiation degradation.
Roofing granules generally consist of crushed and screened inorganic substrate granules which may have their own inherent color. The substrate granules are typically coated with a ceramic coating which may include a pigment.
The useful life of roofing shingles depends upon permanent coverage of the shingle backing by roofing granules. It is thus important that roofing granules be firmly embedded in the adhesive (asphalt in the case of asphalt shingles).
Discoloration of roofing shingles and other building materials due to algae infestation has become especially problematic in recent years. Though the phenomena has traditionally been observed in the south and northwest portions of the United States, its presence has been observed in most areas of the country. Typically, the discoloration is found on the northern slope of a roof where moisture is retained. The infestation may be particularly acute on asphalt shingles. Discolorization has been attributed to the presence of a blue-green algae, Gloeocapsa magma, transported through air-borne particles. The increased usage of calcium carbonate as an asphalt filler in the asphalt shingle manufacturing process is partially responsible for sustained algal growth. Calcium is a macronutrient for Gloeocapsa magma, thus the use of limestone filler provides an unlimited food supply in which the algae can feed. In addition, Gloeocapsa magma is a fairly robust strain of algae which is able to tolerate a variety of climates due to the desiccation properties of the gelatinous sheath surrounding the algae cells.
So-called "tin-acrylate" monomers and polymers were described by Montermoso (J. Polymer Science, 1958, volume 32, pages 523-525). Montermoso describes the synthesis of the trialkyltin ester of methacrylic acid and polymerization of this monomer to form a tin-acrylate polymer.
One of the most extensive uses of this type of polymer has been in antifouling paints such as used to paint ship hulls and the like. Polymers of this type have also been applied to fish nets to provide fouling control, and as protective coatings for wood, latex paints, and plaster. Various polymers, co-polymers, and terpolymers having a plurality of covalently bonded organotin complexes thereon have been described in trade literature. See for example German Democratic Republic Patent Publication No. DD 279258 A1, published May 30, 1990; Japanese Patent Kokai 01193201 A2, published August 3, 1989; and European Patent Application No. EP 342493 A2, published Nov. 23, 1989.
All of the known uses of tin-acrylate polymers are either in alkaline environments such as sea water, or the substrate to which the tin-acrylate polymer is applied is an uncomplicated substrate where there is no problem of adhesion of a granular material to the substrate, such as in the case of roofing granules adhered to asphalt shingles. The inventors herein are not aware of any use of tin-acrylate polymers in an environment where a granular material is to be adhered to a substrate surface which might sustain growth of algae.